Acidic nature of monocarboxylic acids

         (1) Cause of acidic nature

         (i) A atom of carboxylic acid may be shown as a resonance hybrid of the following structures.

            (I)                     (II)

         (ii) Due to electron deficiency on oxygen atom of the hydroxyl group (Structure II), there is a distance of electron pair of O-H bond toward the oxygen molecule. These facilitate the release of hydrogen as proton (H⁺).

(iii) The resulting carboxylate ion also stabilized by resonance (As negative charge is dispersed on both the oxygen atom). It increases the stability of carboxylate anion and makes it weaker base or strong acid.

         (2) Effect of substituent on acidic nature

         (i) An electron withdrawing substituent (- I effect) stabilizes the anion by dispersing the negative charge and therefore increases the acidity.

         (ii) An electron releasing substituent (+ I effect) stabilizes negative charge on the anion resulting in the decrease of stability and thus decreased the acidity of acid.

         Electron with drawing nature of halogen : F > Cl > Br > I

         Thus, the acidic strength decreases in the order :

         similarly :

         (iii) Inductive effect is stronger at a-position than b-position similarly at b-position it is more stronger than at -position

         Example:

         (iv) Relative acid strength in different compounds

•          Greater the value of K_a or lesser the value of pK_a stronger is the acid, i.e. pKa=log Ka
•          Acidic nature (Ka) a 1/molecular weight
•  
•          The formic acid is powerful of all fatty acids.
•          Acetic acid is less weak acid than sulphuric acid due to less degree of ionisation.

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